Divining gold in seafloor polymetallic massive sulfide systems

Abstract

Hydrothermal fluids on the modern seafloor are important carriers of base and precious metals in a wide range of volcanic and tectonic settings. The concentrations and distribution, especially of gold and silver, in associated seafloor massive sulfide (SMS) deposits are strongly influenced by variable source rocks, fluid chemistry, and precipitation mechanisms. Compositional data of 130 SMS deposits around the world show a large range of gold and silver grades, in part reflecting strong buffering of the hydrothermal fluids by their host rocks. Geochemical reaction-path modeling shows that in most cases the investigated hydrothermal fluids are undersaturated with gold and silver, and solubilities can be orders of magnitude higher than the Au and Ag concentrations measured in the corresponding fluids. Precipitation of gold during conductive cooling of mid-ocean ridge black smoker (MOR) fluids occurs at low temperatures but can be very rapid, with > 90% of the gold deposited in the first 25°C of cooling below~150°C. The result is a Zn-Au polymetallic assemblage with Au and Ag deposited at the same time together with Pb and sulfosalts. In ultramafic-dominated (UM) systems, the strongly reduced hydrothermal fluids promote the deposition of gold at higher temperatures and explain the correlation between gold and copper in these deposits. In this case, the lower stability of the AuHS°complex at low ƒO 2 (buffered by fayalite, magnetite, and quartz) results in gold deposition at > 250°C with early bornite and chalcopyrite and before sphalerite and silver, producing a high-temperature Cu-Au assemblage. In sediment-hosted (SED) systems, the much higher pH stabilizes Au(HS) 2 − and keeps gold in solution to very low temperatures, after the precipitation of chalcopyrite, sphalerite, and galena, resulting in Au-poor polymetallic sulfides and very late-stage deposition of gold, commonly with amorphous silica. In arc-related (ARC) systems, gold deposition occurs at somewhat higher temperatures than in the MOR case, in part because the fluids start with higher gold concentrations. This can be explained by probable direct magmatic contributions, and the high ƒO 2 of the fluids, which promotes the solubility of gold at the source. During cooling, gold precipitates at about 160°C with sphalerite, tennantite, silver, and galena, resulting in an Au-rich polymetallic sulfide assemblage. The mixing of hydrothermal fluids with seawater generally causes oxidation and eventually a decrease in the pH at a mixing ratio of 1:1, causing an initial increase in the solubility of gold and silver. This can delay gold deposition from aqueous species to very low temperatures. These complex systematics make prediction of Au and Ag grades difficult. However, important new data are coming to light on the actual concentrations of the precious metals in hydrothermal fluids. In particular, the input of magmatic volatiles and leaching of pre-existing gold can lead to significant increases in the Au and Ag concentrations of the venting fluids and earlier d...

Gold and tin mineralisation in the ultramafic-hosted Cheoeum vent field, Central Indian Ridge

“…In terms of electrum precipitation, an effective mechanism for Au mineralisation at the CVF might have been caused by simple cooling of hydrothermal fluid during the formation of the high-temperature mineral assemblages. This would be consistent with recent thermodynamic modelling that showed Au mineralisation in ultramafic-hosted SMS deposits occurs mainly at high temperatures during conductive cooling of fluid (Fuchs et al 2019).…”

“…In general, these changes result in effective precipitation of Au due to oversaturation of the dominant Au species (e.g. Au(HS) 0 , Au(HS) 2 − , and Pal'yanova 2008;Fuchs et al 2019). Given the incorporation of invisible Au into early-stage sulphide without discrete Au-mineral inclusions, these aqueous Au species were still undersaturated in the fluid during early, lowtemperature mineralisation.…”

Trace Element Signatures in Pyrite and Marcasite From Shallow Marine Island Arc-Related Hydrothermal Vents, Calypso Vents, New Zealand, and Paleochori Bay, Greece

“…The chemical composition of vent fluids varies between island arc, back-arc and mid-ocean ridge hydrothermal systems, which is based on 1) variable contributions of magmatic volatiles, 2) leaching of magmatic host rocks from ultramafic to felsic composition and 3) differences in fluid temperature, water depth and salinity, affecting processes like phase separation (Reeves et al, 2011;Monecke et al, 2014;Kleint et al, 2015;Seewald et al, 2015;Humphris and Klein, 2017;Schmidt et al, 2017). The trace element composition of pyrite is sensitive to changes in fluid composition (Wohlgemuth-Ueberwasser et al, 2015;Keith et al, 2016a;Monecke et al, 2016;Fuchs et al, 2019;Román et al, 2019;Keith et al, 2020), and may therefore preserve a geochemical signature reflecting the tectonic and geodynamic setting of the host hydrothermal system.…”

Anhydrite‐Assisted Hydrothermal Metal Transport to the Ocean Floor—Insights From Thermo‐Hydro‐Chemical Modeling

“…The "pyrite formation potential" is only valid for the assumed initial average EPR fluid composition and closed system conditions. See, for example, Fuchs et al (2019) for information on how initial fluid composition affect precipitation sequences. 5.…”

“…Even though pyrite represents the main sink for Fe, it can also be accommodated in other mineral such as chalcopyrite, isocubanite and magnetite. The abundance of these minerals, their mass fractions, and the timing of mineralization are controlled by the source‐rock geochemistry, the initial fluid chemistry, and the depositional mechanism, for example, changes in T , pH, redox state (Fuchs et al, 2019). To address the complex conditions of pyrite formation, preceding geochemical model simulations including speciation calculations are required.…”